1. Field of the Invention
The invention relates to a miniaturized SQUID module, notably for multi-channel magnetometers, for measurement of varying magnetic fields in a field strength range below 10.sup.-10 T, comprising superconducting and shielded connections between a SQUID chip and a gradiometer.
2. Description of the Related Art
Multi-channel magnetometers are used, for example for function diagnosis of the human brain and heart in order to reproduce the neuro-electric currents. To this end, the small magnetic fields generated by the currents are measured outside the body. These magnetic fields can be as small as 100 fT (100.times.10.sup.-10 T) and can be measured only by means of superconducting components, i.e. SQUIDs (Superconducting Quantum Interference Devices). Customarily, the fields to be measured are picked up by means of so-called gradiometers which are sensitive only to field gradients, but not to uniform fields. Interference signals from remote sources can thus be suppressed. The suppression factor of such gradiometers for disturbing uniform fields, should be smaller than 10.sup.-4. Via twisted superconducting wires, the signal to be measured is applied from the gradiometers to the SQUID so as to be coupled in inductively by means of a coil. Because the SQUID should not respond directly to external magnetic fields but should measure the signals arriving from the gradiometer, the SQUID customarily operates within a superconducting magnetic field shield.
Magnetometer devices of this kind are known, for example from the literature stated below and from EP-A1-0 200 956. The latter document relates to a comparatively complex device for measuring weak magnetic fields, utilizing a plurality of superconducting connections between a SQUID array, arranged on a substrate provided with contact pads, and a gradiometer array which comprises inter alia a rather complex mechanical contact comb for the superconducting connections.
An accurate analysis has revealed that the prior-art superconducting magnetic field shielding itself distorts uniform magnetic fields because of its field displacement, and hence leads to apparent field balancing of the gradiometer even when the gradiometer itself has been optimally balanced by high precision manufacture.
In order to estimate the order of magnitude of the field distortion caused by superconducting SQUID shields, the field balance E can be calculated for a first-order gradiometer having a radius r, caused by a superconducting disc having a radius a which is situated at a distance z.sub.1 and a distance z.sub.2 from the upper turn and the lower turn, respectively, of the gradiometer. According to an article by "J.A. Overweg, M.J. Walter-Peters", Cryogenics, 529 (1978), there is obtained ##EQU1##
By way of example, the field balancing of a superconducting disc having a diameter of 5 mm or 10 mm can be calculated for different distances from a first-order gradiometer having a diameter of 20 mm and a basic length of 50 mm. The result is that a SQUID housing having a diameter of 10 mm should be situated at least 75 mm from the gradiometer in order to ensure that the apparent balancing error remains smaller than 10.sup.-4. These geometrical relationships, however, have substantial drawbacks in view of the required cooling in a cryostat and the arrangement of a plurality of SQUID modules in a multi-channel magnetometer.
Using a formula from the article by "H.J.M. Terbrake et at", IC SQUID 91, pp. 521-524, (1991), laterally offset SQUID shields in a multi-channel system can also be taken into account. For example, for a 19-channel system with a channel spacing of 25 mm it is found that the SQUID modules must be situated at least 150 mm from the gradiometers in the case of a diameter of 10 mm. Thus, these geometrical relationships are even worse in multi-channel magnetometers, ultimately having an adverse effect also on the feasible number of measuring points per unit of surface area.
The SQUID modules also known from articles by "H.E. Hoening et at", IEEE Trans. Magn. MAG17, 2777 (1991), "J. Knuutila et at", Rev. Sci. Instrum. 58, 2145 (1987), "M.B. Ketchen", IEEE Trans. Magn., MAG23, 1650 (1987) and "D.L. Fleming", IEEE Trans. Magn. MAG21, 658 (1985) are of this order of magnitude or are partly even less attractive, because they involve partly a complex SQUID chip assembly, a complex connection of the superconducting wires, a large vessel for superconducting shielding, an unattractive arrangement for connection of the cables to the SQUID electronics, or a large transformer for impedance matching.